1. Field of the Invention
The present invention relates to crystals of (2R,4R)-monatin potassium salt having a superior sweetness.
2. Discussion of the Background
Monatin is a natural amino acid derivative isolated from the root bark of a native plant (Schlerochiton ilicifolius) of northern Transvaal of South Africa, and its structure has been reported by R. Vleggaar et al to be (2S,4S)-2-amino-4-carboxy-4-hydroxy-5-(3-indolyl)-pentanoic acid ((2S,4S)-4-hydroxy-4-(3-indolylmethyl)-glutamic acid) (R. Vleggaar et al., J. Chem. Soc. Perkin Trans., pp. 3095-3098, (1992)). The synthesis of monatin is described in, for example, ZA 88/4220; U.S. Pat. No. 5,994,559; WO03/045914; C. W. Holzapfel et al., Synthetic Communications, 24(22), pp. 3197-3211 (1994); K. Nakamura et al., Organic Letters, 2, pp. 2967-2970 (2000); and the like.
Moreover, in WO03/045914, crystals of the stereoisomer ((2S, 4R) form, the (2R,4R) form, and the (2R,4S) form) of monatin were isolated. It has been disclosed that, of these, the (2R,4R) form of monatin (hereinafter sometimes to be abbreviated as (2R,4R)-monatin) shows high sweetness, and particularly, a crystal of the (2R,4R)-monatin potassium salt is also superior in regard to preservation stability and the like. The crystal of the monatin potassium salt concretely disclosed in WO03/045914 is a crystal of a monopotassium salt, and the crystal shows an X-ray diffraction pattern having characteristic peaks at diffraction angles 2θ of 6.1E, 12.2E, 18.3E, 20.6E, and 24.5E by powder X-ray diffraction (Cu—Kα radiation).
While the above-mentioned salt has superior characteristics in regard to preservation stability and the like as mentioned above, a salt having improved solubility and color stability would be more preferable.